A radar system measures the distance and/or velocity of a target by sensing the effects of the interaction of the target with a beam of either continuous or pulsed electromagnetic energy. In a linear frequency modulated continuous wave (LFM CW) radar system, the target is continuously illuminated with electromagnetic energy, the frequency of which is linearly modulated in time in accordance with a periodic pattern. The radar receiver measures the distance to the target from the difference in frequency between the received and transmitted signals. One problem with LFM CW radar systems results from leakage of a portion of the transmitted energy that is directly coupled to the receiver without first interacting with a target so as to alias as a stationary near range target. The strength of this leakage signal can be sufficiently great that sidelobes thereof mask the target return signals. Radar systems that incorporate a single antenna for both transmit and receive are particularly susceptible to such leakage problems.
Some prior art linear frequency modulated continuous wave (LFM CW) radar systems, including many automotive CW applications, use separate antennas for transmitting and receiving the radar signals. While separate antennas substantially reduces the problem of leakage, the primary difficulty with this method is that the use of separate transmit and receive antenna arrays can prohibitively increase the cost and size of the system.
Other prior art CW radar systems remove the leakage component in the received signal by mixing the received signal with a portion of the transmitted signal that has been shifted in phase by a fixed analog delay line. The analog delay line must be correctly matched to the leakage. The problem with this approach is that the delay of fixed analog delay line is not responsive to changes in the leakage that may result from temperature variations, etc. The problem with analog delay lines is compounded in multiple beam aperture (MBA) architecture radar systems, wherein each beam can have a distinct leakage path and correspondingly requires a separately delayed signal to compensate for the associated leakage. With an increasing number of beams, the corresponding number of delay lines and associated high speed switches--that switch to the correct delay line for the given beam number--can become prohibitively expensive and cumbersome.
Yet other prior art radar systems attempt to reduce the effect of the leakage signal in the final signal processing stage by either heavily weighting the data prior to Fourier Transform processing or else by ignoring the first N range cells of data from the Fourier Transform. The problems with stronger amplitude weighting of the Fourier transform to reduce the sidelobe levels is that this causes significant broading of the peaks, which can reduce the system's ability to recognize closely spaced targets and consequently targets close to the radar system (and the host vehicle). Clearly, the problem with ignoring the first N range cells is that for automotive collision prediction and avoidance radars the near range information is vital in estimating the time to collision, and the likelihood of collision.
Still yet other systems use a pulsed radar rather than a CW radar, whereby the receiver is gated to ignore the leakage signal. When applied to automotive collision prediction, a pulsed radar system requires very short radar pulses (&lt;6 nanoseconds) to detect targets at very near range, which short pulses are difficult to transmit with sufficiently high power to detect far range targets. Accordingly, pulsed radar systems are currently not suited for detecting targets at both near and far ranges as necessary for automotive collision prediction.